A conventional boiling water reactor adopts a so-called jet pump system obtained by combining recirculation pumps installed outside a reactor pressure vessel and jet pumps installed inside the reactor pressure vessel for the purpose of increasing power density.
As illustrated in FIG. 13, a plurality of jet pumps 4 are arranged at equal intervals in the circumferential direction between a rector pressure vessel 1 and a shroud 2 which are vertically installed in a downcomer part 3. As illustrated in FIG. 14 which is an enlarged view of the main part of FIG. 13, the jet pumps 4 each have a riser pipe 5. The riser pipe 5 is fixed to the rector pressure vessel 1 and introduces coolant supplied from a recirculation inlet nozzle 6 of a recirculation pump into the reactor pressure vessel.
A pair of elbows 7A and 7B are connected respectively to the upper part of the riser pipe 5 through a transition piece 14. A pair of inlet throats 9A and 9B are connected respectively to the pair of elbows 7A and 7B through a pair of mixing nozzles 8A and 8B. Diffusers 10A and 10B are connected respectively to the pair of inlet throats 9A and 9B.
In the following description, the inlet throats 9A, 9B and diffusers 10A, 10B are collectively referred to as “inlet throat 9” and “diffuser 10” when they are not differentiated.
Measurement of the flow rate of the jet pumps 4 during normal operation is important for power control of a nuclear power plant. To this end, measurement pipes 11 are provided at the upper and lower parts of each of the diffusers 10A and 10B. The measurement pipes 11 are used to measure the difference in the static pressure between the upper and lower parts of the diffuser 10 during operation, and the obtained measurement value is calibrated with a calibration value that has previously been measured before the use of the plant, whereby the flow rate of the jet pumps 4 are calculated.
Each of the measurement pipes 11 is welded to static pressure holes formed at the upper and lower parts of the diffuser 10 and is welded to be supported by blocks 12 and a support 13 (FIG. 15) which are supporting members fixed to the diffuser 10. As illustrated in FIGS. 16A and 16B, the measurement pipes 11 are arranged in the lower part of the jet pumps 4 in a complicated manner and are connected to pipes outside the reactor through jet pump measurement nozzles 15. The jet pump measurement nozzles 15 are provided at two symmetrical positions in a horizontal cross-section of the reactor pressure vessel 1.
The jet pumps 4 having the configuration described above are exposed to more severe conditions than other equipment because of a high temperature of about 300° C. and flow of a high speed/large flow rate cooling water pumped from a not-illustrated recirculation pumps. Therefore, a large load is applied to each of the members of the jet pumps 4. Especially, the measurement pipes 11 is subject to sever stress as they are affected, either directly or through the blocks 12 and the supports 13, by the fluid vibration generated by the flow of the high speed/large flow rate cooling water of the diffuser 10 pumped from the recirculation pumps. As a result, several pipe breakages have occurred so far. Such a breakage of the measurement pipes 11 makes it impossible to measure the flow rate of the jet pumps 4, posing a problem for the power control of the reactor, so that repair work must be conducted quickly.
As illustrated in FIG. 16B, the measurement pipes 11 are arranged in a narrow annular space 16 between the reactor pressure vessel 1 and the shroud 2. The riser pipes 5, inlet throats 9, and the like are arranged above the measurement pipes 11 as illustrated in FIG. 14. The horizontally extending part (FIG. 15) of the measurement pipe 11 near the supports 13 is closest to the shroud 2, and the interval between the measurement pipe 11 and the shroud 2 at this part is as small as less than 150 mm.
Further, the intermediate body part of the shroud 2 overhangs the horizontally extending part of the measurement pipe 11. This limits much the shape and size of a repair tool for the measurement pipe 11 and a repair method applied to the measurement pipe 11, making the repair work difficult to carry out.
In addition, the site around the measurement pipes 11 are high-radiation area, so that it is very difficult for workers to access the part to be repaired. Therefore, that under present circumstance, there is no alternative way but to remotely carry out the repair work for the measurement pipes 11 from just above the reactor core underwater.
As an example of the repair method for the measurement pipe 11 having the above configuration, there are known a method using a welding machine in an environment obtained by draining reactor water and a method using an underwater laser welding machine (refer to, e.g., Patent Document 1: Japanese Patent No. 4,298,527 and Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2004-209515, the entire contents of which are incorporated herein by reference).
Most breakage events in the measurement pipes 11 described above occur at the welded parts between the measurement pipes 11 and the blocks 12, and there have been proposed only several repair methods that target only the measurement pipes 11 arranged in the vertical direction of the jet pumps 4.
Further, as illustrated in FIGS. 14 and 15, the installation position of the horizontally extending part of the measurement pipes 11 are so narrow that it is difficult to carry out the repair work for this part with the approaches disclosed in Patent Documents 1 and 2.
Furthermore, underwater remote repair work is essential for the horizontally extending part of the measurement pipes 11 due to difficulty in the repair work for positional reasons as described above and further due to the requirement of shortening of the repair process time. Thus, establishment of a repair method that is carried out remotely and underwater has been required for the breakage of the horizontally extending part of the measurement pipes 11.
The present invention has been made in view of the above situation, and an object thereof is to provide a jet pump measurement pipe repair method capable of coping with a breakage event occurring at the lower part of a jet pump at which a measurement pipe extends in the horizontal direction underwater.